This invention relates to integrated circuits and, more specifically, to an optical semiconductor device having, integrally formed therein, a photo diode and a bipolar transistor.
Compared to a hybrid IC combined with a separately produced light receiving element and external circuit elements, a monolithic optical semiconductor device, integrally combining a light receiving element and a peripheral circuit, can be produced at a lower cost and is relatively immune to noise from external electromagnetic fields.
Referring to FIG. 9, a conventional optical semiconductor device is disclosed in, for example, Japanese Laid-open Patent Publication No. 1-205564. A P-type semiconductor substrate 1, has epitaxially grown thereon a P-type epitaxial layer 2. An N-type epitaxial layer 3 is epitaxially grown on epitaxial layer 2. A plurality of P.sup.+ -type separating areas 4 separate epitaxial area 3, and contiguous upper portions of epitaxial layer 2 into isolated islands. A first island is used to form a photo diode 9. A second island is used to form an NPN transistor 10.
Photo diode 9 includes an N.sup.+ -type diffusion area in its upper surface. An N.sup.+ -type buried layer 6 spans the interface between epitaxial layers 2 and 3 in the island forming transistor 10. The portion of epitaxial layer 3 in transistor 10 functions as the collector thereof. A P-type base area 7 is disposed in an upper surface of transistor 10. An N.sup.+ -type N.sup.+ -type emitter area is disposed in an upper surface of base area 7. An N.sup.+ -type type collector contact area 12 is disposed in the upper surface of transistor 10 outside base area 7.
Photo diode 9 is biased to form a PN junction between P-type epitaxial layer 2 and N-type epitaxial layer 3. N.sup.+ -type diffusion area 5 serves as a cathode of photo diode 9. Separating area 4 serves as an anode of photo diode 9.
An accelerated electric field is formed in NPN transistor 10 by an auto-doped layer 11 in epitaxial layer 2 which became autodoped by diffusion of P-type carriers from substrate 1 during epitaxial growth and heat treatment Autodoped layer 11 retards the movement of carriers originating below the depletion region.
To obtain a high speed response of photo diode 9, the depletion region is widened to restrain the movement of carriers occurring outside the depletion region. In the structure in FIG. 9, automatic doping layer 11 overlaps P-type epitaxial layer 2. This overlap results in an increased concentration of impurities and an enlargement of the depletion region.
Epitaxial growth requires processing in a closed vessel into which gasses are fed to grow the desired layer, and to introduce the desired amount of impurities. During growth of P-type epitaxial layer 2, the closed vessel becomes contaminated with the P-type impurities. If an attempt were made to grow N-type epitaxial layer 3 in the same closed vessel used to grow P-type epitaxial layer 2, the P-type contaminants in the vessel would enter epitaxial layer 3 in such amounts that it would be difficult or impossible to attain the desired properties in epitaxial layer 3.
As a consequence of the contamination of the vessel by P-type impurities during epitaxial growth of epitaxial layer 2, the device must be removed physically from the vessel after epitaxial layer 2 is formed, and placed in a clean vessel for growth of N-type epitaxial layer 3. This requirement for removing and reinstalling the workpiece during processing interferes with production. Thus, the same epitaxial growing device cannot be used to produce the prior-art device of FIG. 9.